The present invention relates to a method for selectively and sequentially converting a plurality of signals from one of a digital and analogue form to the other of the digital and analogue form, wherein the signals are of different frequency band widths, so that the signals can be filtered in the same analogue filter in a common analogue path.
Digital video data signals are provided in three commonly used formats, namely, standard definition format, progressive scan format and high definition format. In standard definition format they may be provided in accordance with any of the following standards, PAL, NTSC and SECAM standards, in progressive scan format they may be provided in accordance with 525P and 625P standards, and in high definition they may be provided in accordance with 1080i, 720P and 1250i standards. In general, it is desirable that video signal processing units be capable of processing the video data signals in all three formats. Standard definition format video data signals, in general, are typically of band width of 6 MHz. Progressive scan format video data signals are typically of band width of 12 MHz, while high definition format video data signals are typically of band width of 30 MHz. In general, when being converted from digital form to analogue form, standard definition format video data signals are generally sampled at an over-sampling frequency of between 24 MHz and 29.5 MHz, and typically, at approximately 27 MHz, which is approximately twice the nyquist sampling frequency. Progressive scan format video data signals when being converted to analogue form are sampled generally at a sampling frequency of 24 MHz to 38 MHz, and most commonly at a sampling frequency of approximately 27 MHz, which is approximately the nyquist sampling frequency. High definition format video data signals when being converted to analogue form are sampled at a sampling frequency in the range of 70 MHz to 85 MHz and most commonly at a frequency of approximately 74.25 MHz, which is approximately the nyquist sampling frequency.
At these sampling frequencies, in the conversion of video digital signals in digital form to analogue form, in general, separate analogue reconstruction filters are required for filtering out image frequencies from the analogue signals of the different formats. Such image frequencies are the resultant of the zero order hold characteristics of a DAC, and hereinafter are referred to as image frequencies. Image frequencies occur at the sampling frequency and at multiples thereof. Thus, at a sampling frequency of 27 MHz the image frequencies of the standard definition and progressive scan video signals occur at the sampling frequency of 27 MHz and multiples thereof. Since the frequency of 27 MHz is within the band width of the high definition video signals, a separate analogue reconstruction filter would be required to filter the standard definition signals and the progressive scan signals on the one hand, and the high definition signals on the other hand. Additionally, since the video data signals of each format can require up to three channels, up to nine separate analogue reconstruction filters may be required for filtering the analogue forms of the video data signals in the respective three formats. Analogue reconstruction filters are in general provided as discrete components. Thus, in general, where the digital to analogue converting circuitry for converting digital video data signals to analogue video signals is implemented as an integrated circuit, an appropriate number of output pins are required for providing output signals in the various formats to the respective analogue reconstruction filters. This significantly increases the pin count of the integrated circuit. Alternatively, relatively complex analogue reconstruction filters are required in which the band pass of the filters can be varied for accommodating the video signals of different formats. Such filters require active components which are relatively complex and thus expensive.
Similarly, when analogue video data signals are converted to digital form separate analogue anti-aliasing filters are required for filtering the analogue signals in the respective formats prior to conversion.
There is therefore a need for a method for converting digital video data signals of respective different frequency band widths to analogue form so that the number of analogue reconstruction filters required is minimised. There is also a need for a method for converting analogue video signals of respective different band widths to digital form so that the number of analogue anti-aliasing filters required is minimised. Indeed, there is a need for a method for converting a plurality of signals in one of a digital and analogue form to the other of the digital and analogue form where the signals are of different frequency band widths which facilitates filtering of the analogue forms of the signals in the same analogue filter.
The present invention is directed towards providing such a method.
According to the invention there is provided a method for selectively and sequentially converting a plurality of signals of different frequency band width from one of a digital and analogue form to the other of the digital and analogue form, wherein the signals in the analogue form are passed along a common analogue path through an analogue filter, the method comprising the steps of;
sequentially selecting the signals to be converted,
sequentially passing the selected signals through a converter circuit for converting the signals from the one of the digital and analogue form to the other of the digital and analogue form, and
over-sampling the respective signals in the converter circuit at respective over-sampling frequencies so that the analogue forms of the signals can be filtered in the same analogue filter in the common analogue path.
Preferably, the signal of widest band width is over-sampled at the lowest over sampling rate. Advantageously, the signal of narrowest band width is over-sampled at the highest over-sampling rate. Ideally, the signal of widest band width is over-sampled at a frequency of at least twice the nyquist sampling frequency.
In one embodiment of the invention the signals are digital signals, and the converter circuit is a digital to analogue converter circuit for converting the signals from digital form to analogue form and the analogue filter is a reconstruction filter.
In another embodiment of the invention the over-sampling frequencies for the respective signals are selected such that image frequencies in the analogue signal are sufficiently displaced from the widest frequency band width signal that the image frequencies can be removed from each of the analogue signals by the same analogue filter without affecting the signals. Preferably, the over-sampling frequencies for the respective signals are selected such that the image frequency, the lowest frequency of which is closest to the widest band width signal is displaced from the widest band signal to a frequency at least twice the band width of the widest band width signal.
In another embodiment of the invention the over-sampling frequencies for the respective signals are selected such that the image frequency, the lowest frequency of which is closest to the widest band width signal is displaced from the widest band signal to a frequency at least three times the band width of the widest and width signal.
In a further embodiment of the invention the signals are in analogue form and are converted to digital form, the analogue filter being an anti-aliasing filter.
Preferably, the over-sampling frequencies at which the respective signals are over-sampled are selected such that the analogue anti-aliasing filter can be such as to preserve the signal of widest band width and reject signals of frequencies above the lowest sampling frequency used less the widest band width of the signal sampled at the lowest sampling frequency. Advantageously, unwanted frequencies between the band width of each signal and half the frequency at which it is over-sampled are removed by a combined anti-aliasing filter and decimation fitter.
Preferably, the over-sampling frequencies are selected such that the analogue filter can be provided with a low roll-off rate of attenuation.
In one embodiment of the invention the band width of a first one of the signals is in the range of 4 MHz to 8 MHz, and is over-sampled at a rate of eight times the nyquist sampling frequency.
In another embodiment of the invention the first signal is of band width of approximately 6 MHz
In a further embodiment of the invention the band width of a second one of the signals is in the range of 10 MHz to 18 MHz, and is over-sampled at a rate of four times the nyquist sampling frequency.
In a further embodiment of the invention the second signal is of band width of approximately 12 MHz.
In another embodiment of the invention the band width of a third one of the signals is in the range of 25 MHz to 35 MHz, and is over-sampled at a rate of twice the nyquist sampling frequency.
In a further embodiment of the invention the third signal is of band width of approximately 30 MHz.
In one embodiment of the invention the signals are video signals, and may be in any one or more of the following formats, namely, standard definition format, progressive scan format and high definition format.
In one embodiment of the invention the analogue filter is a second or higher order filter.
The advantages of the invention are many. The main advantage of the invention is that a method is provided for converting signals of different band widths from one of a digital and analogue form to the other of the digital and analogue form, which permits the analogue forms of the signals to be filtered in the same analogue filter. This has the particularly important advantage of reducing the number of analogue filters required for filtering the analogue form of the signal, and thus, where the conversion circuitry is implemented as an integrated circuit chip the pin count of each integrated circuit can be reduced. Additionally, the complexity of the analogue filter required, and indeed, the order of the analogue filter required can also be reduced.
This is a particularly important advantage in the processing of video data signals which in general are required to be provided in three formats, namely, standard definition format, progressive scan format and high definition format, each of which are of different band widths, the band width of the standard definition signals being 6 MHz, that of the progressive scan signals being 12 MHz, and the band width of the higher definition signals being 30 MHz. In the conversion of such video signals from digital to analogue form by displacing the image frequencies of the respective format signals to a frequency sufficiently spaced apart from the widest band width signal, namely, the high definition signal, a single analogue reconstruction filter can be used in each of the channels of the video signals, and the single analogue reconstruction filter is suitable for filtering the signals in the corresponding channel of each of the three format signals. In fact, by over-sampling the standard definition signal at eight times the nyquist sampling frequency, and by over-sampling the progressive scan signal at four times the nyquist sampling frequency, and by over-sampling the high definition signal at twice the nyquist sampling frequency, the image frequency of the video signal which is closest to the widest band width signal is that of the progressive scan format signals, and since this is displaced to a central frequency of 108 MHz, the image frequency of the progressive scan signal is displaced 66 MHz from the high definition signal. This, thus, as well as permitting the analogue signals of the three format video signals to be filtered in the same analogue reconstruction filter, also allows the analogue reconstruction filter to be provided with a relatively low roll-off rate of attenuation.
Similar advantages are achieved when the method is applied to converting video data signals in the three formats from analogue form to digital form, and these advantages will be clear from the description of one of the preferred embodiments of the invention which is given below.
Needless to say, the advantages of the method according to the invention as well as being available to the conversion of video signals from digital to analogue form and from analogue to digital form, are also available in the conversion of other types of signals of different frequency band widths from both digital to analogue form and from analogue to digital form.
The invention will be more clearly understood from the following description of some preferred embodiments thereof, which are given by way of example only, with reference to the accompanying drawings.